1. Field of the Invention.
The present invention relates to positioning a transducer over a disk in a disk file data storage system. More particularly, the present invention relates to demodulation of position information signals provided by the transducer in response to position information read from the disk.
2. Description of the Prior Art.
Disk file data storage systems contain one or more magnetic disks on which data is stored in concentric tracks. A transducer writes, or magnetically encodes, the data on the track. The transducer is also capable of reading the magnetically encoded data from the tracks.
An electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the transducer radially for track seek operations and holds the transducer directly over a track for track following operations.
A servo transducer reads position information from the disk and provides a position signal which is decoded by a position demodulator and presented in digital form to a servo control microprocessor. The servo control microprocessor essentially compares actual radial position of the transducer over the disk with desired position and commands the actuator to move in order to minimize position error.
In one type of servo system, one disk surface is dedicated to contain servo tracks which are encoded with servo position information. The servo position information in the servo tracks is condensed to evenly spaced sectors. A servo transducer flies over the servo sectors as the disk rotates and produces a sampling effect. The actual position transducer information is updated at the end of each servo sector through the use of track identification information and position error information. The track identification number is prewritten into each servo sector and serves as coarse transducer position information. The position error information is written in the servo sector and represents the distance that the servo transducer is located from the center of the track. This position error information serves as fine transducer position information.
The position error information is generally written in two fields. One is referred to as a quadrature field and the second is called a normal field. Position error information obtained from the normal field or the quadrature field is called a normal or quadrature position sample. By decoding the position samples obtained from these two fields, the off-track position of the transducer is determined relative to the center of the track. The position samples are typically decoded by integrating the analog position signal provided by the transducer which represents the position error information magnetically encoded on the disk. The integrated signal is then converted to a digital signal representing transducer position error.
In the past, two separate, theoretically identical, capacitors were used for integrating signals representing the normal and quadrature position samples. However, although the capacitors were theoretically identical, some difference between the capacitance values often existed. This difference produced an error factor between the normal and quadrature samples. Further, since two capacitors were used for integration in some cases, two analog-to-digital (A/D) converters were required to convert the final analog voltage across each of the capacitors. Using two A/D convertors not only increased the cost of the demodulator, but introduced additional errors due to various off-sets in the A/D converters.
Additionally, in the past, the position samples decoded from the quadrature and normal fields were combined in the analog domain to produce a composite position error signal. This technique increased inaccuracies in the demodulator. These inaccuracies were due to the analog circuits which each had associated off-set errors, gain errors, linearity errors and variation with temperature and which were used for combining the position samples.
Further, before integrating the transducer position signal, and between integrations, it is desirable to keep the initial voltage on the integrating capacitors as close to a zero error level as possible.
For these reasons, there is a need for a more accurate servo position demodulator with a null system which compensates for static or time variable DC offsets in the electronics surrounding the capacitor.